You Cannot Put 700MB of Data on a 700MB RW Disc: Why?
You Cannot
Put 700MB of Data on a 700MB RW Disc: Why?
(This
also pertains to DVDs and Solid State Flash Media)
[The
point of this Tip is to help you understand in
the broadest of terms why
you cannot use up all the space on your CD or DVD RW media and your
CD-Rs and DVD-Rs as well as Flash Media.]
Lets
start with RW media where this causes the most confusion:
You
will never be able to format the entire 700MB of any RW media. The
same is true for DVD RW media. This is because the formatting
requires a file allocation table (FAT) be created, the formatted disc
will be segmented into cylinders (circular sections of the disc)
referred to as tracks and that is further divided into Sectors. The
FAT contains columns and rows that containing information on exactly
where data is stored by referencing track and sector within the FAT.
The file may span many sectors and be across several tracks. The data
and the table takes up a lot of room. A good and easy example of this
is a floppy disk. If you have a floppy disk you will notice on the
diskette drawer slide it states it is a 2MB diskette, however, when
formatted you only have roughly 1.44MB of usable addressable space.
The same is true for any formatted RW CD or DVD.
Can
this be Different between Media Maker Companies?
Simply
stated: No.
Just how much space is available after
formatting a RW disc on your system sometimes depends in part on the
software used to format the disc. But this will never be different
enough between media makers to amount to much. The exception is
preformatted discs you may purchase off the shelf. Those preformatted
disc may vary from maker to maker.
How
about Software driven Burner Programs?
In CD RWs, for
example only, the common assumption is that 545MB will be available
for data storage. Every software program may be slightly different;
each software burner producer reserves the same, more or less space
than another. This difference will be slight, if noticeable at
all.
How about
CD/DVD-Rs then?
Additionally, but aside from RW media.
You cannot place 700MB of data on a 700MB CD-R. Room is included on
the media for file sector information and label data for Start of
File and End of File markers. Each piece of data that comprises a
file on the disc, RW or R, has information that is part of a label
such as (although greatly simplified) part 2 of 200. I'm not overly
simplifying this by example but it is close enough for an average
layman to get a grasp of how this works. Each part of a file broken
down by sector size (more on this in the next paragraph) on your disc
has a marker to indicate which part of the whole it comprises. Your
computer uses this data to reconstruct the file when you open it or
copy and paste the file. When moving or sending files these parts of
a file are referred to as packets.
Space
Loss From Sector Size
Another reason for 'space
lossage' on RW media is that when formatted, each sector may be 8K,
16K or 32K in size. However, the last sector in a string of sectors
for a file rarely fills in the full 8 / 16K or 32K of space. Lets
say, for example, that the last sector only has 4K of data and this
is an actual average for your RW disc. You end up loosing 4,12 or 28K
of space for each and every file on the disc. So in reality you have
the 545MB of available space plus the loss of data in the last sector
of a series of file segments. When there are a lot of file written to
a RW disc, the lossage adds up real fast. Does this also apply to
CD-R media? Yes, and can have a dramatic affect. On a 700MB CD-R you
will never be able to put 700MB of data on the disc for this reason.
The
Same is Also True for Hard Drives on your System
Another good example of this loss of
storage size as advertised is with a Computer Hard Drive. A 500GB
Hard Drive will not even come anywhere close to the advertised usable
space once it is formatted, but when you see your drive in the
System, it will be labeled and described as a 500GB drive.
How
about Music CD and DVD MP3s?
Music CDs are quite a bit
different but and not the same for DVD MP3 discs which are data
discs.
When you purchase a CD Disc labeled as a Music CD, it is
preformatted for music only. And I'll describe this method shortly.
And as a result will not store data.
Music tracks on a
Music CD are laid down much like a vinyl album, in one continuous
groove that swirls across the entire disc media. At the beginning and
end of each track on a Music CD is data, called Meta Data that
contains information about the track title and artist/album. As just
described for each track holding data pertaining to track title and
artist/album, you cannot fit exactly 700MB of music you listen to
later. Additionally, the industry standard is to included two seconds
of silence between tracks titles, but still on the same continuous
groove that comprises the entire Music CD.
Is
this True for Solid State Removable Disks like Compact Flash?
Yes,
and for the same reason already stated above pertaining to
CD/DVD-RWs. When the Flash Media is formatted it also contains a FAT
and the media is formatted with Tracks and Sectors. Files written
have to have package labels and are formatted into there sectors and
where the last sector is occupied with data that is usually less than
the sector size and thus it will never be filled up to its maximum
capacity.
[As
stated earlier, the point of this Tip is to help you understand in
the broadest of terms why
you cannot use up all the space on your CD or DVD RW media and your
CD-Rs and DVD-Rs as well as Flash Media.]
Authors note:
As needed, for corrections or additions to this topic I will add
notes to this Tip below as comments.
If you need or wish to
contact me you can reach me from the Ask button found here or at
http://www.fixya.com/users/worldvet/
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